1. Field of the Invention
The present invention relates to an image sensor and a method of manufacturing the image sensor. More particularly, the present invention relates to an image sensor which not only can reduce its thickness without exposing a wire connecting the image sensor and a printed circuit board, but also improve average yield and reliability of products, and a method for manufacturing the image sensor.
2. Description of the Related Art
An image sensor is generally a semiconductor device that converts an optical image into an electrical signal. The image sensor is widely being applied in various fields of our lives. The image sensor includes an optical module portion and a circuit portion. The optical module portion generates electric charges as long as light is received. Also, the circuit portion converts the electric charges into an electrical signal. The image sensor may be classified in a charge coupled device (CCD) and a complementary metal-oxide-silicon (CMOS) device according to a method of embodying the image sensor. The CCD stores and transmits an electric charge carrier in a certain capacity while each MOS capacitor is positioned very near to each other. The CMOS device makes as many MOS transistors as the number of pixels by using a CMOS technology and adopts a switching method of detecting outputs in order by using the CMOS technology. In this instance, the CMOS technology uses a control circuit and a signal processing circuit as a peripheral circuit. The driving method of the CCD is complex and consumes a lot of power. Also, since there are many mask process steps, the process becomes complicated. Also, since a signal processing circuit may not be embodied in a CCD chip, there are some disadvantages such as difficulty of manufacturing as one chip. Accordingly, the CMOS device is currently being studied and used in many cases.
Various manufacturing methods have been developed for the image sensor described above. In particular, methods such as chip on board (COB), chip on film (COF), and the like were usually used in a field where miniaturization is important, such as a camera phone and the like. The COB method first attaches a flexible printed circuit board (FPCB) on the back of an image sensor by using die adhesives and later connects a PCB electrode and an input/output terminal (I/O terminal) of the image sensor by using a gold bonding wire. Namely, the COB method adopts a similar process to existing semiconductor production line. Accordingly, the COB method of manufacturing is highly productive. However, since a wire bonding space is needed, the size of a module becomes larger. Also, the COF method directly attaches a flexible PCB on an image sensor, like the COB method. However, an active side of the image sensor is directly flip-chip bonded to an electrode of the flexible PCB. Accordingly, the COF method does not need a gold bonding wire like the COB method. Also, the height of a lens barrel is lowered. Accordingly, a lightweight, thin, short, and small module may be produced.
However, as a package technology was being improved, a chip size package (CSP) technology was also developed. In the CSP, an image sensor is provided on a glass substrate. While the space between an image sensing portion and an upper glass is empty, epoxy resin is adhered around the image sensing portion and the upper glass. Also, electrical wires are formed on the substrate glass from an I/O terminal of the image sensor. Finally, a solder ball is formed.
After this, the package technology was further developed. A new concept of semiconductor package technology assembling the entire wafer at one time appeared from the form of assembling each chip one by one. The new semiconductor package technology simplified its process and considerably reduced package space. This is called as wafer level package (WLP). Unlike an existing method of individually assembling and packaging chips cut from a wafer, this package technology completes assembling on a wafer where chips are not separated. Namely, this package technology incredibly improved a semiconductor process.
Also, one of the main points to be considered in manufacturing an image sensor is that the image sensor increases a fill factor to improve light sensitivity in a light receiving portion. In this instance, the fill factor is a ratio that the light receiving portion occupies on the entire image sensor. However, in the above method, since a circuit portion may basically not be removed, there is a constraint when an area is limited. Accordingly, a condensing technology appeared that changes the path of light entering an area other than the light receiving portion and condenses the light onto the light receiving portion. For this condensing, the image sensor usually uses a method of forming a micro lens on a light receiving diode.
Conventional art will be described in further detail with reference to FIG. 1. FIG. 1 is a cross-sectional view illustrating a configuration of a camera module including a conventional image sensor. As illustrated in FIG. 1, a camera module 1 includes an optical module portion 10 and a circuit portion 20. The optical module portion 10 includes a lens mount 12. A plurality of lenses 11 are laid layer upon layer in the lens mount 12. The lens mount 12 is connected to a filter mount 14 on the back of the lens mount 12. The filter mount 14 supports an infrared filter 13. The infrared filter 13 is combined with the filter mount 14 and seals the space where an image sensor is provided.
The circuit portion 20 includes an image sensor 21 in the filter mount 14. The image sensor 21 is provided on a PCB 22, and a light receiving portion 25 is provided on the image sensor 21. The light receiving portion 25 includes a plurality of light receiving diodes and a micro lens to improve light sensitivity as described above. In this instance, the micro lens is provided on the upper portion of the plurality of light receiving diodes and condenses light. Also, a conductive wire 23 made of aurum or the like is provided on the outside of the image sensor 21 to electrically connect the image sensor 21 and the PCB 22. The light receiving portion 25 and the image sensor 21 are connected to each other by an internal lead line.
However, a camera module constructed above has following problems.
Firstly, a conductive wire made of aurum or the like is used to connect an image sensor and a PCB creating a long electrical access path. This deteriorates some properties of the image sensor. Also, the image sensor and an infrared filter need to be spaced apart from each other by a predetermined distance, so as to ensure the conductive wire does not make contact with other components. Accordingly, a circuit portion may not be manufactured with less than a certain thickness. Also, this is a limiting factor for making a small-sized camera module.
Average yield decreases because of a process of being exposed to contaminates such as dust or the like. The process may include bonding of a conductive wire. A sealing structure of the image sensor is formed between a filter mount and an infrared filter. Accordingly, an additional process is needed to control an occurrence of contaminates. This increases production cost and deteriorates productivity.
Also, since a PCB and an image sensor are connected to each other by wire bonding, a metal pad has to be installed on the image sensor. Accordingly, a fill factor decreases. Also, the connection between the image sensor and the PCB through wire bonding may be damaged because of a thermal condition occurring in other processes or disconnected or creating an open circuit because of an external impact. These problems may decrease the reliability of the image sensor.
Also, one of the most important problems is that application-specific integrated circuit (ASIC) chips such as a Digital Signal Processor (DSP) are increasingly needed when ASIC chips are 3-dimensionally integrated after being vertically provided on an image sensor. In this case, wire bonding complicates the process and enlarges the size of the image sensor.
The above problems are not solved even when an image sensor is manufactured by using a conventional CSP or WLP. Namely, when a conductive wire and a PCB are connected on the outside of an image sensor, the problems described above remain. That is, since each image sensor chip is cut and wire-bonded by using the WLP, the process causes contaminates, such as dust, to flow into the image sensor. Accordingly, the aforementioned problems caused by wire bonding remain. Recently, a WLP method is being widely used. However, without solving the problems as above, the fast development in the field of an image sensor may not be expected because of the decrease of yield, the limits of reducing the thinness of a circuit portion, the difficulty of 3-dimensional assembly, and the like.